Electric vehicle (EV) development programs are underway in most of the highly industrialized countries. In the U.S., the Department of Energy is managing a $160 million program under the Electric Hybrid Vehicle Research, Development, and the Demonstration Act, passed by Congress in 1976. It is expected that when electric vehicles find widespread use, they will greatly reduce dependence on petroleum and will also have a desirable impact on environmental pollution in urban areas.
The current models of electric vehicles, using lead-acid batteries, offer limited performance that barely qualifies them for short-trip local driving. They can go only about 40 miles before they have to stop for hours-long charging. Speeds are limited to no more than 40-55 mph, and when they are operated at that top speed the batteries are drained even more rapidly. Poor acceleration is another drawback. These limited performance characteristics are further reduced in cold weather. Finally, the batteries available have limited use life with deteriorating performance on repeated charge-discharge cycling. Thus, the key to the future success of electric vehicles is a better battery, e.g., one that weights less, is more compact, stores more energy, releases the energy more rapidly, can be recharged more rapidly, lasts longer, and costs less initially and over its entire life cycle.
A wide variety of batteries have been suggested for vehicle applications, and all have problems to overcome before they will be practical. The leading battery contenders for use in electric vehicles remain the ones that have looked most promising for the past several years, i.e., lead-acid, nickel-iron and nickel-zinc. Of these, the nickel-zinc battery has the best initial energy-to-weight and power-to-weight characteristics. The nickel-zinc battery however exhibits poor cycle lift, i.e., the number of charge and discharge cycles which a battery can undergo before it no longer is capable of performing its intended function.
The poor cycle life of nickel-zinc batteries is especially troublesome when they undergo deep discharges. This problem is associated with any secondary battery which employs zinc as the anode and an alkaline electrolyte, because of the high solubility of the oxidation products thereof, namely, ZnO or Zn(OH).sub.2.
The short cycle life of batteries employing zinc anodes is attributed to premature cell failures which can be characterized as being catastrophic or gradual. Catastrophic cell failures are believed to be due to internal shorting of the cell by the growth of zinc dendrites which form a bridge between the electrodes.
For example, the nickel-zinc battery is based on the following half-cell reactions: ##EQU1##
The reversible reactions are written so that the discharge cycle reads from left to right. The zinc half-cell reaction as written above, however, is an oversimplification since the oxidized form of zinc exists as a mixture of ZnO, Zn(OH).sub.2 and Zn(OH).sub.4.sup.=. The zincate ion (Zn(OH).sub.4.sup.=) is soluble and contributes to the complexity of cell performance.
The nickel-zinc cell discharge reaction supplies a flow of electrons through the external (workload) circuit. To maintain material and electrical balances, however, water and hydroxide ions must flow freely in the electrolyte between the electrodes. In order for the battery to operate effectively, a direct electronic path in the internal circuit between the nickel and zinc must be prevented and yet there must be a path between the electrodes through which the ions can travel which are necessary for the electrode reactions. These two goals are achieved by the use of a separator which is inserted between the electrodes of the battery.
When a battery employing a zinc anode is charged, the above-described reaction reverses and zinc is formed. Ideally, the zinc which is formed is redeposited on the zinc anode. However, some of the zinc which is produced in the charging sequence gives rise to formation of zinc dendrites which tend to bridge out from the zinc anode and connect up with the cathode. Even when a battery separator is inserted between the electrodes the zinc dendrites can actually penetrate the separator over a number of charging cycles leading to catastrophic cell failure.
The gradual, but unacceptable rapid loss of cell energy capacity occurs more frequently with repeated deep discharge cycling wherein the active mass of zinc anode is almost completely depleted. This gradual loss of energy capacity is related to pore plugging and other deterioration in the separator, and to shape change in the zinc electrode. The shape change of the electrode results from the fact that the zinc is not redeposited during charging at the location where it has been oxidized during discharging by accumulates instead in that part of the cell where the current density is greatest causing densification of the electrode.
Many attempts have been made to prevent the formation of dendritic zinc and shape change or to avoid the damaging consequences thereof. Thus, some success has been achieved with a pulsating charging current, electrolyte additives, electrolyte circulation, and the use of special separators.
More specifically, a great deal of attention is being given to the design of battery separators.
As may be gleaned from the above discussion, separator performance is one of the keys to the durability of zinc electrode containing batteries. The separator's ability to control the flow of electolyte components plays a limiting role in determining maximum power to weight ratio, in maintaining a uniform zinc electrode shape, and in retarding the diffusion of zincate to the cathode. The initial electrolyte flow properties should not be altered by the accumulation of ZnO within the pores of the separator. Moreover, the separator is expected to resist the penetration of zinc dendrites which lead to electrical shorts and the separtor material must survive the harsh oxidative alkaline environment of the electrolyte in the vicinity of the cathode for the target life of the cell.
Battery separators employed in the past can be segregated into two basic categories, namely, those which operate by diffusion and those which operate by mass transport.
For example, cellulosic films (e.g., cellophane, sausage casing) have been the most common separators for the nickel-zinc cell. The pore size in cellophane is within the order of molecular dimensions such that transport of water and ions between the electrode compartments is by molecular diffusion. Because of the diffusive transport mechanism, there is an inherent limitation in charging and discharging rates. During charging, local depletions of hydroxide ion occur, leading to electro-osmotic pumping and convective flow of electrolyte, which cause erosion and lateral shape changes on the zinc electrodes.
The most limiting shortcoming of cellulosic separators, however, is their degradation in the cell environment. Oxidation of the cellulose within the cell during discharge results in the formation of CO.sub.2 as one of the products of oxidation, which leads to electrolyte carbonation, lowering of the cell voltage and loss of positive electrode capacity. Eventually the physical failure of the degraded cellulosic separator terminates the cell's life.
Various approaches used to cope with the degradation problem all involve compromises of cell characteristics and/or cost. For example, electrolyte concentrations above 40% KOH are used with cellulosic separators to reduce the degradation rate. However, at 31% KOH, where the cell's internal resistance would be the lowest, the degradation rate of cellophane is unacceptable.
Multiple layers of cellulose separators permit additional cycles, but at increased separator cost and weight gain, and an increase in internal resistance. The same holds true for those which are fabricated with a "protective" layer of a more resistant microporous film shielding the cellophane from the oxygen evolution.
Modified cellulosics have been reported to achieve important effects on cell performance and separator durability. None is offered commercially however. For example, cellophane has been modified by treatment with titanium and cerium. However, the use of titanium obviously increases the cost of the separator to an unacceptable degree.
Microporous polypropylene which has a pore size in the order of 200 A is an example of a separator wherein the electrolyte balance is maintained by mass transport thereof through the pores. Because of the ease of electrolyte transport, concentration gradients do not build up during high rate charge and discharge, and convective flows and electro-osmotic pumping effects are reduced. Furthermore, polypropylene is chemically inert in the cell environment, thus permitting operation at 31% KOH for minimum cell internal resistance. Such mass transport films are not without their own disadvantages, however. For example, the pore structure of certain microporous films permits the transfer of zincate to the nickel compartment. After repeated cycling, zinc and zinc oxide accumulate in the separator. More importantly, however, such microporous films are easily penetrated by zinc dendrites which leads to catastrophic failure of the cell.
One approach towards preventing dendrite shorting of nickel-zinc cells is to provide an auxiliary electrode as illustrated in U.S. Pat. No. 4,039,729. This patent describes a rechargeable galvanic cell wherein an auxiliary electrode is present in the cell and segregated from the positive and/or negative electrode by at least one microporous separator. The auxiliary electrode consists of a porous, electrically conductive material made preferably of copper, iron, or nickel in the configuration of a "netting", "perforated plate", or "screen plate" of a thickness of 0.05 to 0.15 mm. The large thickness of the auxiliary electrode is disadvantageous because it increases the internal resistance of the cell, and reduces the power to weight characteristics of the battery, e.g., the number of cells per unit weight which can be packed into the battery is reduced. Furthermore, the nature of the configuration of the auxiliary electrode requires the use of excessive amounts of nickel which increases the cost of the battery. The use of auxiliary electrodes is also disadvantageous in that present battery manufacturing techniques would have to be significantly modified to provide for placement of an auxiliary electrode between each anode-cathode pair. This does not appear to be a practical or economical solution of the above-described problems.
U.S. Pat. No. 3,970,472 is directed to a rechargeable zinc anode battery which reduces dendrite growth by the use of a dendrite barrier. The barrier comprises an open weave cloth substrate of, for example, random polypropylene fibers, upon which is deposited a porous metal, such as nickel. This dendrite barrier is separated from the zinc anode by consecutive layers of a microporous film, and an electrolyte absorbent layer. The barrier is separated from the cathode by an integral and distinct cellulosic film. The collective battery separator therefore comprises 4 layers and permits electrolyte transport by a diffusion mechanism which increases the internal resistance of the cell. When the nickel is applied to the open weave using an acrylic plastic carrier, the cloth becomes permeated throughout its structure with nickel. This increases the consumption of nickel substantially. Since the effect exerted by the nickel impregnated cloth is to catalyze the oxidation of the zinc dendrites to form a soluble zinc species therefrom, and this effect is a surface phenomenon, i.e., it occurs immediately upon contact of the zinc dendrite with the nickel. The impregnation of the entire cloth with nickel is a waste of nickel. However, due to the open weave nature of the barrier cloth it is not possible to deposit nickel only on the surface thereof in a manner sufficient to assure that the zinc dendrites would not find a path through its interior.
U.S. Pat. No. 3,539,396 discloses a battery separator wherein a layer of sintered metal such as nickel is sandwiched between two membranes of the type made from such materials are cellophane, alumina-silicate ceramic, polyvinyl alcohol and the like. The metal, e.g., nickel, layer is sintered to form an integral and distinct structure in which the metal particles are interconnected to form pores. Alternatively, the nickel particles can be incorporated into a binder such as polypropylene or polyphenylene oxide and in some instances the binder may be interlocked with the metal particles by heat curing. In this instance, the permeability of the barrier is primarily due to the pores of the sintered nickel. The solid polymer binder sintered nickel layer is relatively thick, e.g., 0.001 to 0.020 inch, and therefore inhibits mass transport and increases the internal resistance of the cell as well as leading to electrolytic unbalance.
U.S. Pat. No. 3,539,374 (see U.S. Pat. No. 3,666,517 for the process of making) is directed to a metal-coated plastic substrate such as microporous film which is hydrophobic. The metal coating of the film, however, is applied to the non-porous precursor of the microporous film which is subsequently stretched to develop the open-celled structure therein. More importantly, however, the thickness of the metal film is disclosed as ranging from 0.2 to 20 microns (2,000 to 200,000 A) and no utility of the same as a battery separator is disclosed. Consequently, while such thick metal coatings are useful for providing thermal insulation they would increase the electrical resistance of the film to unacceptable levels.
U.S. Pat. No. 3,793,060 is directed to porous metal coated ultrafine porous polymer articles which can be employed as an electrode or current collector. The pore size of said porous articles is disclosed as having an average diameter of from 40 to 120 A. Such metal coated porous articles are not disclosed as having utility as a battery separator. Consequently, this patent expresses no concern for the criticality of the coating thickness or uniformity and the properties controlled thereby and indeed discloses a thickness of 2000 A and higher. The selection of the metals employed for the coating also reflects a lack of intent that the coated articles be employed as battery separators.
The search has therefore continued for a battery separator which results in improved performance of a zinc electrode containing secondary battery. The present invention has been developed in response to this search.
It is therefore an object of the present invention to provide a microporous membrane having a uniform deposit of a metal having a low hydrogen overvoltage on the surface thereof which is capable of use as a battery separator and is resistant to zinc dendrite penetration.
It is a further object of the present invention to provide a battery separator capable of improving the life cycle performance of a zinc electrode containing secondary battery.
It is still another object of the present invention to provide a battery separator which exhibits improved resistance to internal shorting when employed in a zinc electrode containing secondary battery.
It is another object of the present invention to provide a battery separator which will minimize shape change and densification of a zinc electode when employed in a secondary battery.
It is a further object of the present invention to provide a battery separator which will continue to perform its intended function when employed in a zinc electrode containing battery which is subjected to numerous deep discharge cycles, high charging rates, and accidental overcharges.
It is still another object of the present invention to provide a battery separator which will maintain or improve overall zinc electrode containing battery cell performance parameters including ease of manufacturability, energy density, power density and peak power in a cost efficient manner.
It is a further object of the present invention to provide a zinc electrode containing rechargeable battery which employs a battery separator which achieves the above described objects for a battery separator.
It is still another object of the present invention to provide a process for reducing the penetration of a battery separator by zinc dendrites.
These and other objects and features of the invention will become apparent from the claims and from the following description when read in conjunction with the accompanying drawings.